Hydrazine, monomethyl hydrazine, hydrazinium nitrate, and mixtures thereof have been used, and continue to be used, as monopropellants for rocket engines, gas generators, auxiliary power units (APUs), tank pressurization systems, and other applications. These compounds and mixtures can be catalytically decomposed to produce hot, gaseous products which can then be used to produce thrust, drive a turbine, or otherwise perform work. The advantages of hydrazine and hydrazine derivatives include high performance, fast response time when used with a suitable catalyst, and a well-established record of performance. Furthermore, the decomposition of hydrazine takes place at moderate temperatures (<700° C.), and the decomposition products (N2, H2, and NH3) are not oxidizing. This allows one to use steel and/or nickel-based alloys for the combustion chamber; expensive and exotic materials such as niobium alloys or rhenium are not needed.
Despite their widespread use, hydrazine and hydrazine derivatives are not without drawbacks. Hydrazine, for example, is classified by the Department of Transportation (DOT) as a flammable liquid, a poison, and a corrosive material. It is also carcinogenic and listed in the Environmental Protection Agency's (EPA's) Toxic Substances Control Act (TSCA) inventory. For these reasons, there has been a long-felt need in the chemical propulsion industry for a less-hazardous replacement for hydrazine.
The catalyst most frequently used for decomposition of hydrazine and its derivatives is Shell-405, which is described in U.S. Pat. No. 4,124,538, the entire contents of which is incorporated by reference herein. Shell-405 utilizes highly dispersed iridium on a high-surface area, aluminum oxide support. In a typical satellite propulsion application, the catalyst bed is heated to approximately 200° C. prior to introducing the propellant. Failure to preheat the catalyst decreases the life of the catalyst bed by increasing the severity of the thermal shock experienced by the catalyst due to the large amount of heat released during propellant decomposition. The result of repeated thermal-shock cycles is mechanical attrition of the catalyst granules and expulsion of fines from the bed. Despite the undesirable effects of “cold starts,” the catalyst is capable of decomposing hydrazine at temperatures as low as 2° C., the freezing point of hydrazine. For hydrazine blends with lower freezing points, the catalyst still has sufficient activity to allow cold starts. This capability is useful in satellite applications, as it makes the system usable in the event of failure of the catalyst bed heater.
In the search for a less-hazardous substitute for hydrazine and hydrazine derivatives, the U.S. Army Space and Missile Command identified 2-dimethylaminoethyl azide (also known as DMAZ or CINCH fuel), an organic-azide compound, as a candidate replacement. DMAZ is non-carcinogenic and only one-tenth as toxic as hydrazine. It has a calculated thruster performance comparable to that of hydrazine, and an adiabatic flame temperature slightly less than that of hydrazine. This combination of features make DMAZ attractive as a replacement for hydrazine in virtually all of hydrazine's current applications, including auxiliary power units, emergency power units (EPUs), monopropellant and bipropellant thrusters, tank pressurization systems, and gas generators.
The primary challenge to the use of DMAZ as a hydrazine replacement is the difficulty in catalyzing its decomposition. Shell-405, for example, requires temperatures in the 175-200° C. range to cause rapid decomposition. Although heating the catalyst bed to improve its performance and response time is generally acceptable to the chemical-propulsion and aerospace communities, the mandatory use of such high temperatures is not. A catalyst that requires above-ambient temperatures needs heaters and the associated electronics to power and control said heaters. The resulting system is inherently more complex and prone to significantly diminished reliability.
Azide Chemistry
Organic azides (R—N3) with low molecular weight R groups are notorious for being unpredictably explosive, and their stability is generally increased as the size of the R group increases. More specifically, as the R group becomes more electron-donating, the C—N bond strength—and hence, the stability of the molecule—increases. Consequently, one approach to decomposing organic azides is to use a catalyst that will destabilize the C—N bond by withdrawing electron density from the area. For example, a strong Lewis acid (i.e., a strong electron pair acceptor) will attract electron lone pairs on nitrogen atoms within the azide molecule. The terminal nitrogen is likely the most basic, and thus most likely to be attracted to the Lewis acid. Attraction of one or more of any of the nitrogen atoms will cause the net result of a large withdrawal of electron density from the C—N bond. This will facilitate the first step in the proposed decomposition mechanism cleavage of this C—N bond. Thus, researchers have reported that “Organic azides are sensitized by . . . traces of strong acids.”R—N═N−═N+